Building an FPV Racing Quadcopter, Part 1

By Terry Dunn

This week, we're going to walk through the step-by-step process of building an FPV racing quad and setting it up to fly!

Racing quadrotors have captured the interest of a lot of people. They’re fast, nimble, and tough. Best of all, having a First Person View (FPV) system installed lets you get a sense of what it’s like to be onboard your speed machine. In the past, we’ve presented a video of Norm building a racing quad with the help of Carlos Puertolas (Charpu). We’ve also given you a buyer’s guide that outlined all the equipment you need for your own racing quad. This week, I’ve prepared a four-part series that will cover each aspect of getting a racing quad built and flight-tested:

  • Part 1: Frame Assembly
  • Part 2: Flight Controller Setup
  • Part 3: Configuring the FPV System
  • Part 4: Flight Testing and Tuning

A friendly reminder: if you are new to multi-rotors, racing quads are a horrible place to start. Get yourself something a little more sedate to help you learn the basics. Once you’ve honed your flying skills, racing quads are much more practical and enjoyable.

Frame Assembly

The quad that I’ll be building for this series is a Strider Mini Quad provided by Red Rotor RC. The Strider is a 250mm-class ship with a carbon fiber frame. There are a few features on the Strider that negate purchasing some of the common components found on racing quads. The Power Distribution Board (PDB), lost-model alarm, and On-Screen Display (OSD) are all integrated into the frame itself. This saves you the cost of buying those components separately, as well as the hassle of installing them.

THE STRIDER FROM RED ROTOR RC IS A 250MM RACING QUAD WITH A CARBON FIBER FRAME. AS YOU CAN SEE, THERE AREN’T MANY PARTS. THE INCLUDED HARDWARE HAS BEEN SORTED IN AN ICE TRAY FOR EASY IDENTIFICATION.

Red Rotor provides an online assembly manual, so make sure you are using the latest version. In addition to what’s provided in the kit, you will need a few basic tools and supplies: metric Allen wrenches, zip ties, heatshrink tubing, soldering iron, etc…pretty basic stuff. To prepare for the build, I sorted all of the included hardware in a plastic ice tray. There are four different length screws in the kit and this helped me keep them all distinct.

The first few steps of assembly are very straightforward. They involve fastening the bottom plate of the frame to the center plate. They’re simple assembly tasks with nuts, bolts and spacers. All of the parts lined up perfectly, so things progressed quickly.

A WELCOME FEATURE OF THE STRIDER IS THAT IT HAS SEVERAL ELECTRONIC COMPONENTS INTEGRATED INTO THE FRAME, SO THEY DO NOT HAVE TO BE PURCHASED AND INSTALLED SEPARATELY.

Before mounting the arms to the quad you’ll have to decide whether you will use 5”-diameter props or 6”. This really boils down to the motors you’ve chosen. Each arm has two mounting options to accommodate the appropriate prop. If you are unsure, which prop you’ll be using, just use the 6” locations to mount the arms. You can always move them later. Note that the motor mounting holes on the arms are not symmetrical. You’ll have to pay attention to this to ensure that the wires from the motor are aligned with the arm.

THE STRIDER’S ARMS CAN BE INSTALLED IN EITHER OF TWO CONFIGURATIONS. HERE, THEY ARE INSTALLED IN THE INNERMOST POSITION TO ACCOMMODATE 5” PROPELLERS.

I knew that I would start out using 5” props, so I mounted my arms in the innermost location. It is worth noting that the arms can also be folded to reduce the Strider’s footprint for transport. You just have to loosen two screws for each arm to release them for folding. The same folding action can help absorb energy in a crash to prevent damage to the frame.

Motors

The motors, I chose for this quad are SunnySky 2204 2300kV brushless units from Buddy RC. You may notice that you can buy a “CCW” version of the same motor. The only difference is that the CCW version has reverse threads on the prop shaft. The CCW motor is actually meant to rotate clockwise (as viewed from above). This helps to prevent the aerodynamic drag of the prop from loosening the nut that holds it on. Since you’ll have two motors turning clockwise and two counter-clockwise, you can buy two of each motor type. I just bought four of the standard type, thinking that I’d have to be an idiot to fly with loose prop nuts. Well, it turns out I’m an idiot, but I digress -- more on that when I cover flight testing.

I USED SUNNY SKY 2204 2300KV MOTORS IN THE STRIDER. THE ‘CCW” VERSIONS OF THIS MOTOR HAVE REVERSE THREADS ON THE PROP SHAFT TO HELP PREVENT CLOCKWISE ROTATING PROPS FROM COMING LOOSE. IT’S HANDY, BUT NOT A NECESSITY.

The bolt pattern on the motors fit the holes in the quad arms perfectly. I used the screws included with the motors to mount them to the arms. A drop of blue Loctite thread-locker on every motor screw helps to ensure that none of them vibrate loose.

Electronic Speed Controls

At the same time that I purchased the motors, I also picked up four Velotech Magic 12-amp Electronic Speed Controls (ESCs). The ESCs are available with or without a Battery Eliminator Circuit (BEC). The BEC allows the flight battery to also power the receiver and flight controller (and the servos if any were present). You only need one ESC to BEC. In fact, sometimes having multiple BECs in parallel causes problems. The simplest path is to buy one ESC with BEC and 3 without. Just make sure that they are all the same base ESC.

ON THREE OF THE ESCS, I DISABLED THE BEC CIRCUIT BY REMOVING THE RED WIRE FROM THE RECEIVER PLUG AND INSULATING IT WITH HEATSHRINK TUBING.

All four of my Magic ESCs have BEC, so I disabled it on three of the units. This is easily done by removing the red wire from the ESC’s receiver plug. I used heatshrink tubing to insulate the removed pin on each of the three affected ESCs.

AS SMALL AS THE VELOTECH MAGIC ESCS ARE, THEY WERE STILL A TIGHT FIT IN THE STRIDER.

The Velotech ESCs were a little longer than the units shown in the Strider manual, so they were a pretty tight fit in the indicated locations. I was able to squeeze them into place and secure them with zip ties.

The PDB is integrated into the center plate of the quad. There are solder pads on the top and bottom side of the plate for attaching the power leads of each ESC as well as a pigtail for connection to the flight battery. I decided to place all of the solder connections on the top side of the plate. Knowing that space would be at a premium in this area, I carefully routed each power lead from the ESCs to the intended solder location and cut them to the precise lengths needed.

TAKE YOUR TIME WHEN SOLDERING THE POWER LEADS TO THE PDB. IT DOESN’T HAVE TO BE PRETTY, BUT YOUR JOINTS MUST BE CLEAN AND SOLID.

If you are new to soldering, you may find this job difficult. There are a lot of different wires to manage and a few physical obstacles are in the way. These joints are critical, so it is important to do a good job (no cold joints). Also, since the carbon fiber frame is electrically conductive, you have to make sure that your work is clean with no solder bridges to the chassis. Ask for help if necessary and make sure you get it right. For what it’s worth, I used a 25-watt iron with a wedge tip.

I SHORTENED THE MOTOR LEADS FROM THE ESCS BEFORE SOLDERING, BUT LEFT ENOUGH SLACK TO BE ABLE TO MOVE THE FRAME ARMS TO THEIR OUTER POSITIONS.

You’ll also have to solder the motor leads from the ESCs to the motors. Again, I trimmed the ESC wires so that they could be routed cleanly to the motors. In this case, I cut the wires a little long in case I ever extend the arms to the 6”-prop positions. It’s always better to shorten the ESC wires rather than the motor wires. The motors are wound with magnet wire that has a non-conductive coating on it. If you cut the wires, you have to scrape off this coating before the wire will take solder. It’s a pain.

ONCE THE ESCS ARE IN PLACE, THERE ARE A LOT OF WIRES TO BE DEALT WITH. THEY ALL HAVE A PURPOSE AND A PLACE TO BE.

For each motor, I soldered the three wires but did not insulate them initially. I checked to make sure that each motor rotated in the correct direction first. You could do this by sequentially attaching each ESC to the throttle channel of your radio, but I think it’s easier to use a servo tester. If a motor spun in the wrong direction, I swapped two of the motor leads (any two will do). Once I had the motors spinning correctly, I insulated the solder joints with heatshrink tubing.

Radio Receiver/Transmitter

It takes a 4-channel radio to fly the Strider, but you’ll be better off with at least 6 channels. Those extra two channels allow you to select different flight modes (they define how the quad reacts to control inputs), and also control lights and the alarm.

The example in the directions shows installation of a FrSky Taranis radio system. A lot of flyers like the Taranis for racing quads because it enables a simple 3-wire (CPPM) connection to the flight controller and some receivers feature a built-in RSSI (Received Signal Strength Indication) output. With RSSI, you can overlay a graphic strength indicator on your video screen that tells you if your radio signal is getting weak. That’s handy data for long-distance flyers.

I decided to use my Futaba 7C radio for the Strider. I have nothing against the Taranis, but I’ve used the 7C for years and it’s never given me an ounce of trouble. There’s a lot to be said for confidence in your gear. The tradeoff is that my Futaba R617FS receiver requires a few more wires to connect it to the flight controller, which is mostly a matter of space. While the R617FS does not have RSSI output, it can be added with a relatively simple modification. Since I don’t plan to do any long-distance flights with the Strider, I’m not sure that I’ll ever bother.

The path I’ve chosen requires more wires than other methods. I could reduce wires by using a CPPM receiver and/or trimming the receiver leads from the ESCs. Not only could I have shortened the leads, but I could have completely removed the positive and negative wires from the three ESCs with disabled BECs. Even with all of this extraneous wire, I still did not have any trouble fitting it all within the confines of the frame. In that regard, I think that my build is a worst-case example wiring-wise and it all panned out without any difficulty.

Before mounting the receiver to the frame, I decided to install and configure the flight controller, an Open Pilot CC3D . The Strider frame has nylon studs which are spaced perfectly to hold the CC3D. There is a printed indicator on the flight controller showing which side should face the front of the quad, but this configuration obscured the mini-USB port on the board. So I turned the CC3D 90-degrees and made an adjustment during setup of the board’s firmware to compensate. More about that is coming soon.

Although, there is a slight bit more to complete on the frame, it happens just before flying. So this is a good stopping point. In Part Two, I will cover wiring and configuration of the CC3D flight controller.

Terry spent 15 years as an engineer at the Johnson Space Center. He is now a freelance writer living in Lubbock, Texas. Follow Terry on Twitter: @weirdflight